Processor for use as a path searcher of a spread spectrum receiver and a method of operation of the processor

ABSTRACT

A path searcher for use in a spread spectrum receiver, including a correlator for correlating a received signal in different time positions with a reference code to select for further processing time positions of the signals showing the strongest correlations, wherein the correlator is operable to obtain a correlation energy result for each time position in a set of possible time positions, a comparator for comparing the correlation energy results with each of a plurality of energy thresholds and a recorder for recording data indicating time positions for correlation energy results which are not less than the respective thresholds in different groups corresponding to the respective thresholds. Also described is a semiconductor device and a receiver incorporating the path searcher and a method of operation of the path searcher.

FIELD OF THE INVENTION

The present invention relates to a processor for use as a path searcherof a spread spectrum receiver and a method of operation of theprocessor. In particular, the processor may provide a stage of amulti-stage path searcher of a channel profile acquisition stage in aWCDMA receiver.

BACKGROUND OF THE INVENTION

Third Generation mobile communication systems are modern systemsdesigned for multimedia communications. Using such systems,person-to-person communications can be established which include highquality picture and video images. Access to information and services onpublic and private networks in enhanced form can also be achieved by thehigher data rates and improved data handling capabilities availableusing such systems.

WCDMA (Wideband Code Division Multiple Access) technology has emerged asthe most widely adopted air interface technology for Third Generationmobile technology. Such technology is based upon protocols defined in anumber of industry standards. A number of the important internationalstandards have been defined as 3GPP (3rd Generation Partnership Project)standards by ETSI (the European Telecommunications Standards Institute).

In a WCDMA system, all users employ the same carrier frequency in agiven channel and all can transmit or receive simultaneously. In orderto be able to distinguish between different users, each transmittinguser in each channel uses a code sequence which is unique, the‘scrambling code’. Each channel carrier has a large bandwidth (e.g. 5MHz), and transmitted user information bits (symbols) for radiocommunication are spread over a wide frequency bandwidth of the channelby multiplying the user data bits with a spreading code comprising asequence of ‘chips’ or rectangular pulses having a regular chip length.

In a receiver for use in a WCDMA system, the Channel Profile Acquisition(CPA) stage is a very important stage for decoding a received signal.Such a stage includes a processor known as a path searcher. The purposeof this processor is as follows. In wireless communication, signals maytravel along many different propagation paths before reaching areceiver. Since each path has a different length and the signals travelat the same speed, there is multiplicity of arrival times at thereceiver for the signals from the various paths. The signals at thedifferent arrival times are replicas of one another differing only inamplitude and phase. The path searcher finds the signals from differentpaths received at different arrival times.

As defined by 3GPP, the path searcher in one known form may be based onthe Primary Common Pilot Channel (P-CPICH) standard in which thetransmitted signal may be considered as a P-CPICH signal. According tothe P-CPICH standard, each communication frame has a duration of 10 msecin a common timing protocol of the communication system, and each framecontains 15 slots. In each slot, ten complex symbols of 1+j aretransmitted. Each symbol is expanded into chip level by a fixedspreading factor of 256 with a spreading code of “all ones.” P-CPICHsignals carry data (user communicated data), spreading code, andscrambling code. However, data is in the form of the complex symbol 1+jand the spreading code consists of 256 ones, so the only valuableinformation extracted from P-CPICH signals is therefore the scramblingcode. The path searcher performs a scrambling code correlation with areceived signal at each time position of a given set of possiblepositions to select the positions giving the strongest correlationresults in terms of correlation energy.

In order to reduce processing complexity of a processor serving as apath searcher, a procedure known as a double dwell search may beemployed. Such a procedure operates as follows. In a first dwell orfirst stage of processing in the procedure, a short duration correlation(with a given scrambling sequence) is carried out in each of a completeset of possible signal time positions for a given channel. A sub-set ofthe time positions showing the strongest correlations is selected forfurther processing in a second dwell or second stage of processing inthe procedure. In the second dwell, a longer duration correlation iscarried out for each of the time positions selected by the first dwellto select an even smaller sub-set of the time positions for furtherprocessing.

SUMMARY OF THE INVENTION

In accordance with aspects of the invention, there is provided aprocessor for use in a path searcher of a spread spectrum receiver, asemiconductor device incorporating the processor, a receiver includingthe processor and a method of operation of the processor as defined inthe appended claims.

Specific embodiments of the invention are set forth in the dependentclaims. These and other aspects of the invention will be apparent fromand elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details, aspects and embodiments of the invention will bedescribed, by way of example only, with reference to the drawings.

FIG. 1 is a block schematic diagram of part of an illustrative WCDMAreceiver embodying the present invention.

FIG. 2 is a flow chart of a method of operation in a path searcher ofthe receiver of FIG. 1.

FIG. 3 is a schematic graph of correlation energy versus timeillustrating operation of a path searcher of the receiver of FIG. 1operating in accordance with an embodiment of the present invention.

FIG. 4 shows two tables illustrating processing of data in a pathsearcher of the receiver of FIG. 1 operating in accordance with anembodiment of the invention.

FIG. 5 is a graph of data throughput versus data channel energy ratio,illustrating relative simulated performance of processors embodying theinvention compared with processors of the prior art for use as pathsearcher in the receiver of FIG. 1

FIG. 6 is a block schematic diagram of an example of arrangement offunctional units which may be employed in the path searcher of thereceiver of FIG. 1.

DESCRIPTION OF EMBODIMENTS

Examples of embodiments of the invention will be described in terms of asystem-on-chip (SoC) comprising part of a WCDMA receiver including apath searcher of the receiver. Although the examples will be describedin terms of a system-on-chip (SoC) comprising part of a WCDMA receiverincluding a path searcher, it will be appreciated that the inventiveconcept herein described may be embodied in any apparatus or device thatincorporates part of the WCDMA receiver including the path searcher.

Referring to FIG. 1, there is illustrated an architecture for part of aWCDMA receiver 100. The receiver 100 is illustratively an equalizerbased receiver, although for a different implementation it could be inthe form of a rake receiver instead. The receiver 100 may include anantenna 101, a front end processor 103, an ADC (Analog to DigitalConverter) 105, a cell searcher 107, a path searcher 109, a channelestimator 111, a memory 112, and equalizer 113 and a despreader 115. Anincoming received RF signal is received by the antenna 101. The signalis processed in analog RF form by the front end processor 103 which mayinclude for example one or more low noise amplifiers, one or more AGC(Automatic Gain Control) processors and a frequency downconverter whichconverts the signal to a baseband form. The signal is converted todigital form by the ADC (Analog to Digital Converter) 105. The ADC 105produces an output signal which comprises the received signal inbaseband digital form. The output signal from the ADC 105 is deliveredto each of several components of a demodulator line-up which maycomprise the cell searcher 107, the path searcher 109, the channelestimator 111, the equalizer 113 and the despreader 115.

The cell searcher 107, the path searcher 109, the channel estimator 111,the equalizer 113 and the despreader 115 all perform known demodulationfunctions.

The cell searcher 107 finds information relating to the identity andsynchronisation of the system base station cell which forwarded thereceived signal. In particular, the cell searcher 107 finds a scramblingcode associated with the received signal and provides the code as aninput to the path searcher 109.

The path searcher 109: (i) searches through the arrival positions intime of the replica signals from each of the different possiblepropagation paths which together constitute the wanted signal; and (ii)selects the positions in time which give the strongest correlationresults in terms of correlation energy. The path searcher 109 may be asdescribed in more detail later with reference to FIGS. 2 to 4 and FIG.6.

The channel estimator 111 performs very long correlations (up to 7680chips in duration) for the replica signals at each of the time positionsselected by the path searcher 109 to estimate the amplitude and phase ofthe signals at those selected time positions. The selected timepositions of the replica signals are thus equivalent to channel taps orfingers.

The equalizer 113 is a filter that reconstructs signals destroyed by thecommunication channel. The equalizer 113 calculates weights andpositions of coefficients based on the time positions selected by thepath searcher 109 and whose amplitude and phase have been found by thechannel estimator 111. The equalizer 113 has a response which convergesapproximately to the inverse of the channel impulse response.

The despreader 115 performs correlation of the reconstructed, spreadsignal (at the chip rate) with the CDMA spreading code to convert thesignal from its chip rate to its symbol rate to extract the symbolscommunicated as user information in the received signal.

The memory 112 stores programs and data needed in operation by themodulation and decoding components shown in FIG. 1, especially the pathsearcher 109. The memory 112 may store the scrambling code found by thecell searcher 107 and provided to the path searcher 109 for use by thepath searcher 109.

The path searcher 109, optionally together with any one or more of thecomponents of the receiver 100 shown in FIG. 1, may comprise asystem-on-chip (SoC) or other implementation in which the components areincorporated in a semiconductor device (integrated circuit chip).

The path searcher 109 will now be described in more detail. The pathsearcher 109 may illustratively be based on the Primary Common PilotChannel (P-CPICH) standard referred to earlier. For each time positionof a set of time positions corresponding to different signal propagationpaths, the path searcher 109 carries out a correlation with a receivedsignal at the position to find the time positions giving the strongestcorrelations. It is to be noted that the received signal at each timeposition may consist of a combination of many codes of different datachannels, which may include the wanted P-CPICH channel, as well asnoise. The unknown signal at each time position is correlated with thewanted scrambling code. The wanted scrambling code is the scramblingcode of the wanted cell having the reference scrambling code found atthe cell search stage by the cell searcher 107. The wanted scramblingcode is only one of many possible codes that may have combined to givethe total received signal at each time position. So the purpose of eachcorrelation is to filter out or weaken unwanted codes to isolate andstrengthen the wanted code. This allows time positions that includereplicas of the wanted signal to be found, based on the knowledge thatthese replicas include the wanted scrambling code.

The path searcher 109 performs a path searching procedure 200 which (inits most general form) is known per se and is illustrated in FIG. 2. Thepath searcher 109 performs the procedure 200 repeatedly for eachprocessing frame of the received signal, that is each cycle of operationof the path searcher 109. The duration of the path searching procedure200 (in terms of a system clock controlling timing within the receiver100) is dependent on the hardware implementation employed, so theduration and frequency of the procedure 200 can be selected to suit thepractical hardware implementation of the receiver 100. For example, thepath searching procedure 200 can be performed for each consecutive WCDMAframe, where the length of the WCDMA frame is 10 msec. Alternatively,the path searching procedure 200 can be performed for example for eachconsecutive WCDMA slot, wherein there are 15 slots in one WCDMA frame.

The path searching procedure 200 carried out in each processing framemay be a double dwell procedure which includes steps 201 and 203relating to a first dwell and steps 205 and 207 relating to a seconddwell of the double dwell procedure.

In the step 201, the path searcher 109 performs a short durationcorrelation operation with the signals received at each of a completeset A of different possible time positions in a given channel.

The set A may for example comprise m equally spaced time positions,where m is a large number which is an integral power of two, e.g. 512(2⁹). Each correlation is carried out at each of the time positionsusing a code stored in the memory 112. The code used for thecorrelations may be the scrambling code for a P-CPICH signal referred toearlier as found by the cell searcher 107. The correlations in step 201may be carried out as serial or as parallel operations or some of theoperations may be carried out serially on parallel blocks of thecomponents. The correlations in step 201 are of short duration. Eachcorrelation may last for example for a time equivalent to 256 (2⁸) or512 (2⁹) chips. The number of results obtained in step 201 equals thenumber of time positions in set A, i.e. one result is obtained for eachtime position. The best results obtained in step 201 are those obtainedfrom wanted replica signals which provide the strongest correlations asshown by the greatest correlation energies as obtained (by squaring)from the respective correlation amplitudes.

In the step 203, a sub-set B of the results (also known as ‘taps’) ofthe short correlations carried out in step 201 is selected. The sub-setB consists of a maximum number n of the best results obtained in step201, where n is a number smaller than m which is an integral power oftwo, e.g. n is 64 (2⁶). An example of a procedure which may be used tocarry out step 203 is as described later with reference to FIGS. 3 and4.

In the step 205, the replica signals at the selected time positionsgiving the sub-set B of the best correlation results in step 203 arefurther processed. Each of the replica signals is subject to a furthercorrelation with the same code used for the short duration correlationsin step 201. The correlations in step 205 may be carried out as serialor as parallel operations or some of the operations may be carried outserially on parallel blocks of the components. The correlations in step205 are of longer duration than those of step 201. Each correlation instep 205 may last for example for a time equivalent to 2048 (2¹¹) or4096 (2¹²) chips. The longer duration correlations allow more accuratecorrelation energy results to be obtained compared with the coarsershort duration correlations of step 201. The best results obtained instep 205 are again those results obtained from replica signals whichprovide the greatest correlation energies as obtained from thecorrelation amplitudes.

In the step 207, a sub-set C of the time positions of replica signals isselected from the results of the correlations carried out in step 205.The sub-set C is a sub-sub-set of the sub-set B. The sub-set C mayconsist of a maximum of p results obtained in step 205, where p is anumber smaller than n which is an integral power of two, e.g. p is 16(2⁴) or 32 (2⁵).

The sub-set C of results is delivered for further processing to thechannel estimator 111 via the equalizer 113 and further processed in aknown manner.

FIG. 3 is a graph 300 of correlation energy versus time illustrating howstep 203 may be carried out by an example of a procedure. Fourillustrative correlation results 301, 303, 305 and 307 are shown forfour different time positions within the set A of m time positionsreferred to above. Each result is shown as a vertical line whose upperend represents the correlation energy result. The correlation energy isgiven by the square of the correlation amplitude. Each result iscompared with each of a plurality of threshold correlation energyvalues, namely a threshold T1, a threshold T2 which is at a greaterenergy level than the threshold T1, a threshold T3 which is at a greaterenergy level than the threshold T2 and a threshold T4 which is at agreater energy level than the threshold T3. The result 301 is seen to begreater than the threshold T1 but less than the thresholds T2, T3 andT4. The result 303 is seen to be greater than all of the thresholds T1,T2, T3 and T4. The result 305 is seen to be greater than the thresholdsT1 and T2 but less than the thresholds T3 and T4. The result 307 is seento be greater than the thresholds T1, T2 and T3 but less than thethreshold T4.

Data entries for the time positions giving correlation energy resultswhich are not less than the four respective thresholds T1, T2, T3 and T4are recorded in four different groups. Each data entry may identify thetime position of the replica signal giving a particular result as wellas the correlation energy result. A first group G1 consists of dataentries for the correlation energy results which are not less than thethreshold T1 but less than the other thresholds. A second group G2consists of data entries for the correlation energy results which arenot less than the threshold T2 but less than the thresholds T3 and T4. Athird group G3 consists of data entries for the correlation energyresults which are not less than the threshold T3 but less than thethreshold T4. A fourth group G4 consists of data entries for thecorrelation energy results which are not less than the threshold T4.

The different groups G1, G2, G3 and G4 referred to above may be compiledin one or more tables or lists. Where tables are employed, they may forexample include: (i) a first table in which are recorded (entered) thedata entries for the first group G1; (ii) a second table in which arerecorded the data entries for second group G2; (iii) a third table inwhich are recorded the data entries for the third group G3; and (iv) afourth table in which are recorded the data entries for the fourth groupG4. Thus, the number of tables in which data is recorded may be equal tothe number of thresholds used as well as the number of groups of dataentries which correspond to those different thresholds.

Thus, using the four tables described above, data for the replica signalshowing the result 301 is recorded in the data entries for that group G1in the first table; data for the replica signal showing the result 303is recorded in that for the group G4 in the fourth table; data for thereplica signal showing the result 305 is recorded in that for the groupG2 in the second table; and data for the signal showing the result 307is recorded in that for the group G3 in the third table.

Where there are four tables to accommodate the four groups G1, G2, G3and G4 of data entries as described above, each of the tables may have amaximum of n/4 data entry positions to fill. Alternatively, one or moreof the tables may have a maximum of more or less than n/4 positions tofill. In any event, the tables have a maximum of n positions in total tofill.

The selection of results for further processing in step 205 is made foreach processing frame using the four tables by taking the data entriesfrom the different tables, beginning with the entries for the group G4in the fourth table, then the entries for the group G3 in the thirdtable, then the entries for the group G2 in second table, then theentries for the group G1 in the first table.

The example of a method for use in step 203 of the procedure 200illustrated in FIG. 2 may be operated using a minimum of two thresholds,thereby forming a minimum of two groups, although any number ofthresholds greater than two, e.g. four as illustrated in FIG. 3, may beused instead, thereby forming a corresponding number of data entrygroups.

A further example is illustrated in FIG. 4. The further method is amodification of that using four tables described above. In the furthermethod, the four thresholds shown in FIG. 3 are employed to form datagroups in two tables. In the further method, the four thresholds shownin FIG. 3 are again used to form four data entry groups G1, G2, G3 andG4. However, a single Table 1 is employed to record: (i) the group G1 ofdata entries which are for results which are not less than the thresholdT1 only; and (ii) the group G2 of data entries which are for resultswhich are not less than the threshold T2 but less than T3 and T4. Inaddition, a single Table 2 is employed to record the group G3 of dataentries which are for results which are not less than: (i) thethresholds T1, T2 and T3 but not T4; and (ii) the group G4 of dataentries which are for results which are not less than the threshold T4.The Table 1 and the Table 2 have a total of n data entry positions.

The Table 1 and the Table 2 employed in the further method describedabove may have an equal number n/2 of data entry positions.Alternatively, the Table 1 and the Table 2 may have unequal numbers ofdata entry positions wherein the relative numbers are selected bydesign.

FIG. 4 shows two tables illustrating processing of data using the twoTables 1 and 2 in the further example of a method. Data entry positionswhich appear in the upper part of the Table 1 are indicated in FIG. 4 bythe reference numeral 401. Data entry positions which appear in thebottom part of the Table 1 are indicated in FIG. 4 by the referencenumeral 403. Data entry positions which appear in the upper part of theTable 2 are indicated in FIG. 4 by the reference numeral 405. Data entrypositions which appear in the bottom part of the Table 2 are indicatedin FIG. 4 by the reference numeral 407.

In the further method, the group G1 of data entries may be formed inTable 1 shown in FIG. 4 by filling the data entry positions in Table 1beginning at the uppermost position 401 and working downward to filleach vacant position in turn. The group of data entries G2 may be formedin Table 1 by filling the data entry positions beginning at the lowestposition 403 and working upward to fill each vacant position in turn.The group G3 of data entries may be formed in Table 2 by filling thedata entry positions beginning at the uppermost position 405 and workingdownward to fill each vacant position in turn. The group of data entriesG4 may be formed in Table 1 by filling the data entry positionsbeginning at the lowest position 407 and working upward to fill eachvacant position in turn. Thus, in the further method, a data entry forthe replica signal showing the result 301 in FIG. 3 is recorded in thegroup G1 of data entries in the Table 1 shown in FIG. 4. A data entryfor the replica signal showing the result 303 in FIG. 3 is recorded inthe group G4 of data entries in the Table 2. A data entry for thereplica signal showing the result 305 in FIG. 3 is recorded in the groupG2 of data entries in the Table 1. A data entry for the replica signalshowing the result 307 in FIG. 3 is recorded in the group G3 in theTable 2.

If, eventually, all of the data entry positions in the Table 1 becomefilled in a given processing frame, the group G1 and the group G2 willmeet. This may be at the centre of the Table 1 or above or below thecentre. Thus, in filling of the Table 1, some of the data entries of thegroup G2 may be made in positions 401 in the upper part of the Table 1or some of the data entries of the group G1 may be made in positions 403in the lower part of the Table 1, depending on the relative numbers ofentries included in the groups G1 and G2.

Similarly, if, eventually, all of the positions in the Table 2 becomefilled in a given processing frame, the group G3 and the group G4 willmeet. This may be at the centre of the Table 2 or above or below thecentre. Thus, in filling of the Table 2, some of the data entries of thegroup G4 may be made in positions 405 in the upper part of the Table 2or some of the positions 407 in the lower part of Table 2 may be filledby data entries of the group G3, depending on the relative numbers ofentries included in the groups G3 and G4.

When the Table 1 incorporating data entries for the group G1 and for thegroup G2 has become filled, and at least one further result is obtainedrequiring a data entry in the group G2, then the data entry in the groupG2 may be made by replacing a data entry already in the Table 1 in thegroup G1. Similarly, when Table 2 has become filled and at least onefurther result is obtained requiring a data entry in the group G4, thenthe data entry in the group G4 may be made by replacing a data entryalready in the Table 2 in the group G3. The replaced entry in the groupG3 may itself be saved by moving the entry to the Table 1, e.g. byreplacing an entry in the lowest energy group G1. The selection ofresults for further processing in step 205 is made for each frame usingthe Tables 1 and 2 by taking the data entries from the Tables 1 and 2 toidentify the time positions of the selected replica signals, beginningwith the entries for the Table 2, then the entries for the Table 1.

Using combined tables to accommodate multiple groups of data entries inthe manner described above with reference to FIG. 4 allows the pathsearcher 109 to be implemented more easily in hardware form and allowsoperation of the path searcher 109 to be managed more easily than in theembodiment in which each data entry group is compiled in its owndedicated table, e.g. in four tables using four thresholds.

Where only two thresholds are employed in operation of the step 203, thedata entries for the results selected using the two thresholds areinserted into two groups corresponding to the two thresholds. The groupsmay be incorporated in a combined single table or list or in separatetables or lists for each group.

One or more of the thresholds employed may in use be dynamicallyadjusted to suit operating conditions. For example, one or more of thethresholds may be set at a variable level at the start of eachprocessing frame of the path searcher 109. (As noted earlier, in thiscontext, a processing frame is the duration of one cycle of operation ofthe path searcher 109.) For example, at least one of the thresholds foruse in a given processing frame may be set at an energy level whichrelates to one of the correlation energy results for the precedingframe. For example, the thresholds for use in each processing frame maybe set at levels which relate to a maximum energy result Emax for thepreceding frame. The thresholds may be set at pre-determined energydifferences relative to the maximum energy result Emax for the precedingframe. The thresholds may be set at energy levels which differ inpre-determined steps (in energy) from Emax.

For example, where the units of the correlation energy results are −dBand the four thresholds illustrated in FIG. 3 are used, the lowest(smallest) energy threshold T1 may be suitably set at a level which is12 dB below Emax, the energy threshold T2 may for example be suitablyset at a level which is 9 dB below Emax, the energy threshold T3 may forexample be suitably set at a level which is 6 dB below Emax, and thehighest (greatest energy) threshold T4 may for example be suitably setat a level which is 3 dB below Emax. Where the units of the correlationenergy results are −dB and the minimum number of two thresholds areused, the lower (smaller) energy threshold may for example be suitablyset at a level which is 10.4 dB below Emax and the higher (greater)energy threshold may for example be set at a level which is set at alevel which is 4.7 dB below Emax. These various levels were found to besuitable by design using trial and error.

It is to be noted that, in operation of step 203 of the path searcher109, the number n, e.g. n=64, is the maximum number of the results thatcan be selected for use in step 205 in a given processing frame. Allresults which are selected for data entry in one of the tables referredto above will have a correlation energy which is not less than at leastthe first (lowest energy) one of the thresholds, e.g. the threshold T1in FIG. 3. If the number of results that are not less than at least thefirst (lowest energy) threshold for the frame is less than n then thatis satisfactory, since all results having an energy less than (smallerthan) the first (lowest energy) threshold are considered as noise. Ifthere are more than n results for the processing frame that are not lessthan the first (lowest energy) threshold, then when n of the resultshave been obtained and the related data has been entered in the tablesas described earlier, the additional results obtained following the nresults may be ignored (unless they can replace a result in a lowerenergy group as described earlier) since there is no place in the tablesfor them. That is satisfactory since results which are not less than atleast the first (lowest energy) threshold are already available.

Sorting correlation energy results in the order of their energies is aprocedure known in the prior art. However, this known ‘set ordering’ or‘ordering’ procedure involves ordering the correlation results for allof the m members of the set A of possible components. In order to carryout such ordering using such a large number m of results, m·log 2msorting operations are required. In contrast, the example methodsembodying the invention can be operated using only m·k operations, wherek is the number of thresholds used. Thus, the methods embodying theinvention can be operated using a significantly smaller number ofprocessing operations. This allows the processor serving as the pathsearcher 109 to require less MCPS (Mega Cycles Per Second) in softwarecapacity and a smaller hardware logic size and power consumption. Asnoted earlier, the processor, which may be a digital signal processor inthe form of a semiconductor device (integrated circuit), canbeneficially be fabricated using a smaller die size. The processorserving as the path searcher 109 may also carry out functions of one ormore of the other processing components shown in FIG. 1 (or otherfunctions of the receiver 100 which are not shown in FIG. 1) as well asthat of the memory 112.

In the prior art it is also known to build a table using a single energylevel threshold to select correlation energy values for furtherprocessing. However, such a selection method is not satisfactory if thethreshold is not appropriately set. If the threshold is set too low, toomany results will be selected which are noise rather than signal. On theother hand, if the threshold is set too high, the selection method isnot satisfactory because there are too few results which are above thethreshold. In contrast, operation of step 203 in accordance with themultiple threshold procedure embodying the invention allows thethresholds to be set so that the results may be selected in a simplemanner by use of the multiple thresholds without having too many resultswhich are for noise and too few results because one threshold is set toohigh. The multiple thresholds may be set at suitable energy intervals tocover the range of energy results obtained.

Thus, the multiple threshold selection procedure embodying the inventioncan give a better performance than the known single threshold selectionprocedure. The performance may be slightly inferior to that of the knownset sorting procedure but is less complex to implement and can givesatisfactory results approaching those obtained using the set sortingprocedure. The relative performance is illustrated as follows withreference to FIG. 5.

FIG. 5 is a graph 500 illustrating the performance of the multiplethreshold procedure embodying the invention relative to that of theknown path searcher sorting (‘sorting’) procedure referred to earlierand the known ‘one threshold’ procedure referred to earlier. The graph500 shows results for simulation carried out for a WCDMA receiver havingthe line-up shown in FIG. 1 when processing a P-CPICH signal. The firstdwell in the path searcher 109 produces 512 (five hundred and twelve)short correlation results from which no more than the 64 (sixty four)best results are to be selected in the step corresponding to step 203.In the simulation, two examples of the multi-threshold method embodyingthe invention were used. In a first example using four thresholds,namely the four thresholds T1, T2, T3 and T4, the threshold T1 was setin each processing frame at a level 12 dB below Emax for the precedingframe, the threshold T2 was set in each processing frame at a level 9 dBbelow Emax for the preceding frame, the threshold T3 was set in eachprocessing frame at a level 6 dB below Emax for the preceding frame andthe threshold T4 was set in each processing frame at a level 3 dB belowEmax for the preceding frame. In a second example using two thresholds,the lower (smaller energy) threshold was set in each processing frame ata level 10.4 dB below Emax for the preceding frame, and the higherenergy threshold was set in each frame at a level 4.7 dB below Emax forthe preceding frame.

In the graph 500, data throughput measured in Mbps (Megabits per second)is plotted against data channel energy as a ratio of total cell energymeasured in dB. A curve 501 plotted in the graph represents the resultsobtained using the known path searcher sorting procedure. A curve 503represents the results obtained using the multiple threshold procedureembodying the invention using four thresholds. A curve 505 representsthe results obtained using the multiple threshold procedure embodyingthe invention using two thresholds. A curve 507 represents the resultsobtained using the known single threshold procedure. The curve 507 wasplotted using for each frame a single threshold 9 dB below Emax for thepreceding frame (The figure of 9 dB was the best figure selected fromvarious different figures tried).

For each curve plotted in FIG. 5, the higher the curve for a given ratioof data channel energy the better the performance. In other words, thehigher the curve, the greater the amount of data that can be transmittedfor the same data channel energy. As can be seen from FIG. 5, each ofthe multiple threshold procedures represented by curves 503 and 505gives a significant improvement in performance compared with the singlethreshold procedure represented by curve 507. Each of the multiplethreshold procedures represented by curves 503 and 505 gives aperformance close to the known sorting procedure represented by curve501 but the sorting procedure requires a more complex implementation.

FIG. 6 illustrates an arrangement 600 of functional units which is anexample of an arrangement which may be employed in the path searcher 109to carry out the procedure 200 in one of the ways described above, e.g.with reference to FIGS. 3 and 4. The arrangement 600 includes acorrelator 601 which receives an input signal to be processed. Thecorrelator 601 carries out the short dwell correlation described earlierat each of the time positions in the set A of time positions using thereference code previously obtained from the cell searcher 107. Thereference code is supplied to the correlator 611 by a reference codesupplier 611 which may itself have obtained the code from a memory whichhas stored the code since its receipt from the cell searcher 107, forexample the memory 112. The correlator 601 produces for each of the Atime positions a correlation energy result. A comparator 603 compareseach of the A correlation energy results with each of the requiredmultiple thresholds provided by a multiple threshold setter 613. Arecorder 605 records a data entry for each of the correlation energyresults which are not less than the respective thresholds. Each dataentry includes the time position giving the particular result. Therecorder 605 records the data in groups according to the differentthresholds that the various results equal or exceed as describedearlier. In some cases, the recorder 605 may replace an entry in a lowerenergy group with one in a higher energy group as described earlier.

At the end of a processing frame processed by the arrangement 600 whencorrelations have been carried out by the correlator 601 for all of theA time positions and data for the corresponding results have beenrecorded, a reader 607 reads out the recorded data relating to the n orless results which have been recorded by the recorder 605. Thus, thecomparator 603, the recorder 605 and the reader 607 may be considered tocomprise a selector which selects the sub-set B of time positions givingthe best correlation energy results in the first dwell correlationprocedure operated by the correlator 601.

An output signal produced by the reader 607 is delivered to a furthercorrelator 609 to begin a second dwell correlation procedure. The outputsignal indicates the sub-set B of the corresponding selected timepositions giving the data entries recorded by the recorder 605. Theoutput signal produced by the reader 607 is also delivered to themultiple threshold setter 613 which uses the data included in the signalproduced in each processing frame in one of the ways described earlierto set the multiple thresholds for use in the next processing frame ofthe comparator 603.

The correlator 609 carries out a longer dwell correlation for each ofthe sub-set B of time positions as described earlier using the referencecode supplied by the reference code supplier 611. A recorder 615 recordseach of the correlation energy results produced by the correlator 609. Asorter 617 sorts the correlation energy results recorded by the recorder617 in terms of their correlation energies. As will be apparent to thoseskilled in the art, the operations of the recorder 615 and the sorter617 may be carried out as separate operations or combined into a singleoperation. A selector 619 selects from the results sorted by the sorter617 the best p results and produces an output signal accordingly whichindicates a sub-set C of the time positions having the best p resultsselected by the selector 619. The output signal produced by the selector619 is delivered for further processing in a known manner to the channelestimator 111 (FIG. 1) via the equalizer 113 (FIG. 1).

As will be apparent to those skilled in the art, the operations of thevarious units shown in FIG. 6 may be carried out as separate operationsalthough, in some cases, the operations of some units shown, such as therecorder 615 and the sorter 617, may be combined into a singleoperation.

It will be understood that the improved path searcher and method ofoperation therefor, as described above, aims to provide at least one ormore of the following: a performance which approaches that of a pathsearcher using the known sorting procedure but which may be implementedand operated in a simpler manner; and a performance which is better thanthat of a path searcher using the known one threshold procedure.

Also, the path searcher embodying the invention, and optionally one ormore other components of the receiver in which the path searcher isincorporated, e.g. the cell searcher 107, may be applied by asemiconductor manufacturer to any integrated circuit architecturesuitable for implementing such a path searcher. It is further envisagedthat, for example, a semiconductor manufacturer may employ the pathsearcher in a design of a stand-alone device, or application-specificintegrated circuit (ASIC) and/or any other sub-system element.

It will be appreciated that any suitable distribution of functionalitybetween different functional units or controllers or logic elements ormemory elements, may be used without detracting from the embodimentsherein described. Hence, references to specific functional devices orelements are only to be seen as references to suitable means forproviding the described functionality, rather than indicative of astrict logical or physical structure or organization. By way of example,the cell searcher 107 coupled to the path searcher 109 have beenillustrated and described as separate functional elements of thereceiver 100. However, the cell searcher 107 coupled to the pathsearcher 109 may be provided within a combined functional element.

Aspects of the invention may be implemented in any suitable formincluding hardware, software, firmware or any combination of these. Theelements and components of an embodiment of the invention may bephysically, functionally and logically implemented in any suitable way.Indeed, the functionality may be implemented in a single unit or IC, ina plurality of units or ICs or as part of other functional units.

Furthermore, it is contemplated that the path searcher 109, e.g. in theform of the arrangement 600, may be a programmable device which isoperated by application of a suitable computer program. Thus, there maybe a computer-readable storage element in one of the forms well known tothose skilled in the art which has computer-readable code stored thereonfor programming signal processing logic to perform a method ofprocessing a signal in a path searching procedure in a spread spectrumreceiver, the method being to operate the path searcher 109 in one ofthe ways described earlier.

Although the present invention has been described in connection withsome embodiments, it is not intended to be limited to the specific formset forth herein. Rather, the scope of the present invention is limitedonly by the accompanying claims. Additionally, although a feature mayappear to be described in connection with particular embodiments, oneskilled in the art would recognize that various features of thedescribed embodiments may be combined in accordance with the invention.In the claims, the term ‘comprising’ does not exclude the presence ofother elements or steps.

Furthermore, although individual features may be included in differentclaims, these may possibly be advantageously combined, and the inclusionin different claims does not imply that a combination of features is notfeasible and/or advantageous. Also, the inclusion of a feature in onecategory of claims does not imply a limitation to this category, butrather indicates that the feature is equally applicable to other claimcategories, as appropriate.

Furthermore, the order of features in the claims does not imply anyspecific order in which the features must be performed and in particularthe order of individual steps in a method claim does not imply that thesteps must be performed in this order. Rather, the steps may beperformed in any suitable order. In addition, singular references do notexclude a plurality. Thus, references to ‘a’, ‘an’, ‘first’, ‘second’etc. do not preclude a plurality.

The invention claimed is:
 1. A path searcher for use in of a spreadspectrum receiver, comprising: a correlator to correlate a receivedsignal in different time positions with a reference code to select forfurther processing time positions of the signals showing the strongestcorrelations, wherein the correlator is operable to obtain m correlationenergy results, one for each time position in a set of m possible timepositions; a comparator to compare the m correlation energy results witheach of a plurality of energy thresholds to separate the m correlationenergy results which are not less than the respective energy thresholdsinto groups; a selector to select for further processing a sub-set of upto n signals defined by the m correlation energy results, including arecorder to record data to indicate time positions for the separatedcorrelation energy results in the groups, to indicate a first group thathas an energy ordering according to the highest of the thresholds, toindicate a second group that has results that are not less than at leasttwo of the thresholds, and to indicate a third group, that has resultswhich are not less than at least one of the thresholds; and a reader toread results recorded in each of the first, second and third groups inturn in the energy ordering of the groups beginning with the group ofhighest energy until a predetermined number of results have beenobtained or, if there are less than the predetermined number of resultsrecorded in the groups, until all results in the groups have beenobtained.
 2. The path searcher according to claim 1 wherein the recorderis operable to record the results in a number of the groups greater thantwo.
 3. The path searcher according to claim 1 wherein the recorder isoperable to record data for the results for each of the different groupsin a table dedicated to the group, the number of tables being equal tothe number of groups.
 4. The path searcher according to claim 1 whereinthe recorder is operable to record data results for each of a pluralityof the groups in a combined table, the number of tables being less thanthe number of groups.
 5. The path searcher according to claim 4 whereinthe recorder is operable to record data results for a first group in thecombined table by filling positions in turn from one end of the tableand to record results for a second group in the combined table byfilling positions in turn from the other end of the table.
 6. The pathsearcher according to claim 5 wherein the recorder is operable when thetable is full to replace a data entry for at least one result for thefirst group with a data entry for at least one result for the secondgroup.
 7. The path searcher according to claim 1 wherein the recorder isoperable to record in a first table data for correlation energy resultsnot less than a first energy threshold and less than a third thresholdand to record in a second table data for correlation energy results notless than a third threshold.
 8. The path searcher according to claim 7wherein the recorder is operable to record in the first table also datafor correlation energy results not less than a second threshold greaterthan the first threshold and less than the third threshold.
 9. The pathsearcher according to claim 8 wherein the recorder is operable to recordin the second table also data for correlation energy results not lessthan a fourth threshold greater than the third threshold.
 10. The pathsearcher according to claim 1 wherein the recorder is operable to recordfor each data entry for a given correlation energy result the energyresult as well as a time position of the signal having the correlationenergy result.
 11. The path searcher according to claim 1 furthercomprising a threshold setter for adjusting one or more of the energythresholds dynamically to suit operational conditions.
 12. The pathsearcher according to claim 11 wherein the threshold setter is operableto adjust one or more of the thresholds at the start of each processingframe of operation of the path searcher.
 13. The path searcher accordingto claim 12 wherein the threshold setter is operable to set one or moreof the thresholds at the start of each processing frame of operation ofthe path searcher to an energy level which relates to one of thecorrelation energy results for the preceding frame.
 14. The pathsearcher according to claim 13 wherein the threshold setter is operableto set one or more of the thresholds at the start of each processingframe of the path searcher at an energy level which relates to themaximum correlation energy result Emax for the preceding frame.
 15. Thepath searcher according to claim 14 wherein the threshold setter isoperable to set one or more of the thresholds at the start of eachprocessing frame of the path searcher at a pre-determined energydifference relative to the maximum correlation energy result Emax forthe preceding frame.
 16. The path searcher according to claim 1 whereinthe correlator is operable to correlate the signal at each of the timepositions in the set of time positions with a reference scrambling codefound in an earlier operation by a cell searcher.
 17. The path searcheraccording to claim 1 further comprising a first correlator to carry outa first correlation procedure and a second correlator to carry out asecond correlation procedure in each processing frame of the pathsearcher, wherein the first correlator is operable to correlate areceived signal at each position of a set of time positions with areference code, and the second correlator is operable to correlate areceived signal at each position of a sub-set of time positions selectedfrom the set of time positions, the correlation by the second correlatorbeing for a longer duration than the correlation by the firstcorrelator.
 18. A spread spectrum receiver including the path searcheraccording to claim
 1. 19. A method of processing a signal in a pathsearching procedure in a spread spectrum receiver comprising:correlating by a correlator signals received from different paths atdifferent positions in time with a reference code; selecting for furtherprocessing time positions of n of the signals showing the strongestcorrelations, including obtaining m correlation energy results, one foreach time position in a set of m possible time positions of the signals,comparing each of the m correlation energy results with each of aplurality of energy thresholds and separating correlation energy resultswhich are not less than the respective energy thresholds into groups;recording data indicating time positions for the separated correlationenergy results in groups, indicating a first group that has which groupshave an energy ordering according to the highest of the thresholds,indicating a first group that is not less than at least two thresholds,and indicating a third group, that is not less than at least one of thethresholds; and reading results recorded in each of the first, secondand third groups in turn in the energy ordering of the groups beginningwith the group of highest energy until a predetermined number of resultshave been obtained or, if there are less than the predetermined numberof results recorded in the groups, until all results in the groups havebeen obtained.
 20. A non-transitory computer-readable storage elementhaving computer-readable code stored thereon for programming signalprocessing logic to perform a method of processing a signal in a pathsearching procedure in a spread spectrum receiver, the methodcomprising: correlating signals received from different paths atdifferent positions in time with a reference code selecting for furtherprocessing time positions of n of the signals showing the strongestcorrelations, wherein the selection is carried out by obtaining mcorrelation energy results, one for each time position in a set of mpossible time positions of the signals; comparing each of the mcorrelation energy results with each of a plurality of energy thresholdsand separating correlation energy results which are not less than therespective energy thresholds into groups; recording data indicating timepositions for the separated correlation energy results in groups,indicating a first group that has an energy ordering according to thehighest of the thresholds, indicating a second that is not less than atleast two of the thresholds, and a third group that is not less than atleast one of the thresholds; and reading results recorded in each of thefirst, second and third groups in turn in the energy ordering of thegroups beginning with the group of highest energy until a predeterminednumber of results have been obtained or, if there are less than thepredetermined number of results recorded in the groups, until allresults in the groups have been obtained.